Sing-around method, phase-difference method, time-difference method are wellknown techniques for measuring the velocity of flow and/or the flow rate of clear water. Particularly, the time-difference technique has been frequently used. This system typically includes a pair of transducers disposed obliquely in a facing relationship and positioned upstream and downstream in the path of travel of the liquid so that the velocity vector of the liquid changes the propagation speed of the ultrasonic wave along a path from the transmitter to the receiver transducer.
In one mode, the upstream-positioned transducer transmits ultrasonic signals to the downstream-positioned transducer to obtain the time for the acoustic wave traveling or being propagated downstream while in another mode, the downstream-positioned transducer transmits ultrasonic signals to the upstream-positioned transducer to obtain the time for the acoustic wave traveling or being propagated upstream. The velocity of the liquid is determined from the time difference between the upstream and downstream modes of propagation.
However, any of the above-mentioned three techniques is a transmission method in which it is assumed that ultrasonic waves can travel through the liquid medium between the sound transmitter and receiver and reach the latter without significant loss or attenuation.
Air bubbles and solids contained in the liquid are obstacles to ultrasonic waves and scatter them. For this reason, the transmission method is applicable only to clear water which contains few or no air bubbles and/or solids.
The Doppler shift or reflection method is also utilized to measure the velocity and/or the flow rate of fluid. In a typical Doppler shift ultrasonic flow meter, an acoustic transmitter and receiver are mounted on the opposite sides of a test conduit in which fluid flows. The transmitter transmits ultrasonic waves which then travel, for example, upstream with respect to the direction of the fluid containing air bubbles and/or solids, and are scattered by those particles. Part of the scattered or reflected waves comes into the field of the receivers and is observed as a Doppler-shifted signal. The value of the Doppler shift relates to the velocity or average velocity of those particles in the fluid near the central axis which have received the transmitted waves and reflected them toward the receiver. It is clearly understood that this method cannot be applied to flow measurement of clear water which does not include air bubbles or solids.
Although the existence of particles or reflecting objects is a pre-requisite for the Doppler-shift flow measurement, the prior art Doppler-shift flow meters are not applicable to very polluted water containing a large number of particles.
There exists a great need for continuous measurement of flow of a liquid in which pollution degree varies with time, such as the industrial waste water of the type for which the total amount of pollutants is regulated. To achieve this in the prior art, at least two types of flow measuring equipment are required. This complicates the transportation and installation work. Further, the operator must monitor the level of pollution and decide which type of the equipment to switch into operation. This requires high skill on the part of the operator.